As examples of a thin film-forming method used in manufacturing a semiconductor device, methods such as physical vapor deposition (PVD) such as sputtering or the like and chemical vapor deposition (CVD) using a chemical reaction may be utilized. In forming a silicon nitride film (SiN(Si3N4)) through CVD, there has been well-know CVD methods using SiH2Cl2 and NH3. In recent years, there is also used a film-forming method in which a silicon nitride film is formed by alternately supplying SiH2Cl2 and NH3. In these film-forming methods, by heating an entirety of a processing chamber, a substrate to be processed within the processing chamber is heated to a desired temperature for forming a silicon nitride film. Thus, while the silicon nitride film is formed on the substrate, the processing chamber is also heated to substantially the same temperature so that a silicon nitride film is also formed on walls of the processing chamber. Therefore, as the process for forming the silicon nitride film is repeated, a cumulative film of SiN is formed on the inner walls of the processing chamber. If a thickness of such cumulative film exceeds a certain level, the cumulative silicon nitride film may begin to separate from the walls. This may cause particles to be generated in the processing chamber. It is assumed that the separation of the cumulative film is mainly caused by a difference in thermal expansion coefficients between the silicon nitride film and the processing chamber walls or a stress generated within the silicon nitride film.
If the cumulative silicon nitride film begins to separate, a cleaning operation for removing the cumulative film is performed in order to reduce the particles. As examples of the cleaning operation, there have been known a wet cleaning method in which a film-forming apparatus is first stopped from operation and target components thereof are immersed in a chemical liquid such as a hydrofluoric acid or the like to remove the cumulative film through an in-liquid reaction, and a dry cleaning or gas cleaning method may be utilized in which a cleaning gas is introduced into a processing chamber, as in the film-forming process, to remove the cumulative film through a reaction of the film with etching species activated by heat or the like (see Patent Document 1).